Implement the window::fullscreen_mode event

Patch status: merged

Patch by Tony Crisci

Long description:

The fullscreen_mode event is a window with the "change" property set to
"fullscreen_mode". This event should be emitted whenever a window enters
or exits fullscreen mode.
This event can be used to turn off dpms off when a window is fullscreen
or display the fullscreen container name in the status line for
instance.